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Thread: Electromagnetic spectrum questions

  1. #1 Electromagnetic spectrum questions 
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    1. I've seen statements that infrared (heat) waves travel at the same speed as visible (light) waves through space. Is that correct? How is the speed measured?

    2. Do the other waves (radio, gamma, etc.) travel at the same speed?

    3. The "Solar Constant" measures the amount of heat energy delivered to a square foot of material set perpendicular to the sun's rays for one hour at the outer edge of the earth's atmosphere.
    What comparative amounts would be delivered to Mercury and Venus?
    What properties of infrared waves explain the difference?

    4. Are the heat waves distinct and separable from light waves? It seems that they're the same, as when you feel cooler when moving from a sunlit area to a shaded area.

    Thanks,
    George


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  3. #2 Re: Electromagnetic spectrum questions 
    Reptile Dysfunction drowsy turtle's Avatar
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    Quote Originally Posted by gs99
    1. I've seen statements that infrared (heat) waves travel at the same speed as visible (light) waves through space. Is that correct? How is the speed measured?
    Yes, this is correct. 'c' is the speed fo any electromagnetic radiation through a vacuum. It would be measure by the time taken for photons to travel from an emittor to a detector, which are seperated by a known distance.

    Quote Originally Posted by gs99
    2. Do the other waves (radio, gamma, etc.) travel at the same speed?
    All electromagnetic waves, yes.

    Quote Originally Posted by gs99
    3. The "Solar Constant" measures the amount of heat energy delivered to a square foot of material set perpendicular to the sun's rays for one hour at the outer edge of the earth's atmosphere.
    What comparative amounts would be delivered to Mercury and Venus?
    They would have an increased value.

    Quote Originally Posted by gs99
    What properties of infrared waves explain the difference?
    All EM radiation follows the inverse square law, meaning that as the distance from the source increases, the intensity of the radiation from it decreases, by an inverse square.



    Quote Originally Posted by gs99
    4. Are the heat waves distinct and separable from light waves? It seems that they're the same, as when you feel cooler when moving from a sunlit area to a shaded area.
    They are similar to light, but have a longer wavelength, and less photon energy. We feel infrared as heat, but not light.

    Try standing under an energy-saving lightbulb; the light won't make you warmer at all. Sunlight, we can conclude, contains infrared light, therefore.


    "The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at or repair." ~ Douglas Adams
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  4. #3 Re: Electromagnetic spectrum questions 
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    Quote Originally Posted by gs99
    1. I've seen statements that infrared (heat) waves travel at the same speed as visible (light) waves through space. Is that correct? How is the speed measured?

    2. Do the other waves (radio, gamma, etc.) travel at the same speed?

    3. The "Solar Constant" measures the amount of heat energy delivered to a square foot of material set perpendicular to the sun's rays for one hour at the outer edge of the earth's atmosphere.
    What comparative amounts would be delivered to Mercury and Venus?
    What properties of infrared waves explain the difference?

    4. Are the heat waves distinct and separable from light waves? It seems that they're the same, as when you feel cooler when moving from a sunlit area to a shaded area.

    Thanks,
    George
    drowsy turtle's explanation using electromagnetic waves is correct.

    You can also explain these phenomena in terms of photons, the quanta of light.

    It is all a matter of photons, all forms of electromagnetic waves are photons. They differ in frequency.

    1. Photons travel at one and only one speed. That speed is denoted as "c" and is about 3x10^10 cm/s. This is called the speed of light in a vacuum. It can be measure by determining the time between emission of photons at a source and receptin at a detector. That is how time domain reflectometers work in the fiber optics industry, with corrections for the speed of light through glass. The speed of light in a medium occurs through many absorptions and re-emissions of photons. These absorptions and re-emissions take a little time, so the apparent speed is somewhat less than in a vacuum, but the photons themselves always travel at "c" between atoms.

    2. All elelctromagnetic waves are photons. They differ iun the energy per photon.. The energy in a photon is proportional to its frequency. Infrared photons have low frequency (longer wavelength) than visible light photons. But it is all photons and they all travel at one speed.

    3. The power delivered in phton flux is proportional to the frequency of the photons and the number of photons received over a unit area per unit time. Because the surface area of a sphere is the flux density decreases like

    4. There is really no such thing as a "heat wave". Heat is something a bit different and is really the kinetic energy of molecules in both translation and vibratory modes. Energy associated with these molecules can be emitted and absorved when electrons change states and emit photons or absorb photons. That can occur at any wavelength, but commonly is in the infrared region. Then it is just a bunch of photons. You feel cooler in the shade because your body is absorbing fewer photons than when it is in direct sunlight.
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  5. #4  
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    Thanks for that info.

    All EM radiation follows the inverse square law, meaning that as the distance from the source increases, the intensity of the radiation from it decreases, by an inverse square.
    My example:
    If at 1 unit the intensity is 256,
    the intensity at 2 units is 64 (256/2 squared = 256/4)
    the intensity at 3 units is 28.44 (256/3 squared = 256/9)
    Is the math correct?

    But what determines the "1 unit" dimension?
    Doesn't it make a difference if "unit" is mm, mile or AU?

    George
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  6. #5  
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    Quote Originally Posted by gs99
    But what determines the "1 unit" dimension?
    Doesn't it make a difference if "unit" is mm, mile or AU?
    No, because the energy you receive is determined by the ratio of the distances squared you compare. Ratios of the same quantity are free of units. In detail:

    AU (distance between sun and earth)
    AU (distance between mercury and earth)

    The radiation power of the sun is given as one solar luminosity (). At a position with a distance from the sun, the solar constant is given as

    ,

    where is the unit area on what you measure the incoming flux (square metre, square foot, etc.). The ratio between the solar constants of mercury and earth is then:


    So, the solar constant on mercury is 6.67 times higher than on earth. No units involved. The result does not change, if you replace the distances or areas with any other unit you choose.
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  7. #6  
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    Thanks for that info.

    Your explanation provides the intensity ratio of two distances.
    But I do not understand all of your equation.

    Could the same answer be provided by this simpler equation:
    (NEAR distance/FAR distance)^2 = FAR intensity decrease
    This follows the law's verbiage (distance increases, intensity decreases)

    Example comparing Mercury and Earth:
    (.387/1)^2 = .387^2 = Earth intensity is .15 x Mercury
    Confirm:
    Mercury solar constant (9138 W/m^2) x .15 = 1370 for Earth

    or the opposite direction
    (FAR distance/NEAR distance)^2 = NEAR intensity increase

    Example comparing Earth and Mercury:
    (1/.387)^2 = 2.583^2 = Mercury intensity is 6.67 x Earth
    Confirm:
    Earth solar constant (1370 W/m^2) x 6.67 = 9138 for Mercury

    It works OK with Earth/Venus (1/.723 = 1.91; 1.91 x 1370 =2617)

    I'm obviously at a beginner stage; this ratio calculation is sufficient to answer my question. Appreciate your steering me in the right direction.

    George
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  8. #7  
    Moderator Moderator Dishmaster's Avatar
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    Quote Originally Posted by gs99
    Could the same answer be provided by this simpler equation:
    (NEAR distance/FAR distance)^2 = FAR intensity decrease
    This follows the law's verbiage (distance increases, intensity decreases)
    Yes. This is exactly expressed by the equation:

    Quote Originally Posted by Dishmaster
    It means that the ratio of the two irradiation levels is given by the inverse ratio of the distances squared. I just showed you, how the ratio is derived and why it is squared, and not just linear. It goes like this:

    First you need the energy output of the sun in all directions, called luminosity. Then you want to know, how much of this is absorbed by a certain unit area, called A in this example. The fraction of of the energy absorbed is the total energy multiplied with the fraction of A compared to the surface area of the sphere at the location of a planet (earth or mercury, etc.). This gives you the solar constant at that location. The remaining thing to do is to derive the ratio of the solar constants of different distances. Since those distances are the only thing that are variable, all the other quantities collapse to unity. The neat thing is that the final calculation turns out to be independent of the solar luminosity and the unit area A. Therefore, their values are not needed.
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  9. #8 Re: Electromagnetic spectrum questions 
    Time Lord
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    Quote Originally Posted by gs99

    4. Are the heat waves distinct and separable from light waves? It seems that they're the same, as when you feel cooler when moving from a sunlit area to a shaded area.
    People often confuse Infrared radiation with "heat waves", because most warm objects emit Infrared light. If you're standing near a hot furnace, and you feel like heat is reaching you directly from it (instead of just the air around you getting warm), then it's because the furnace is emitting a very strong intensity of Infrared light in your direction. You can't see it with your eyes, but you will feel it on your skin.

    If it's glowing red hot, that's because it has gotten so hot that it's now emitting visible light too.
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